Bought and finished the main sp story of Micromon(Released July 31st) over the weekend.
Bought it for $1.29 it does have IAP(Diamonds) but only once was I really hinted into buying them, when you get asked to enter a special area the first time(You wouldn't have enough diamonds through normal play to enter at that moment).
I don't feel IAP would play a role if your just looking for general play. You can get a limited amount of diamonds in game. I finished the game with plenty of gold and only did mild amounts of grinding and only really used 2 high level Micromon throughout as they level quite quickly especially when 1 can solo a tamer.
Pic of battle and my Wind starter
Some differences from Pokèmon is there's a stamina based sprint and dash ability, tamer won't engage you on sight and you can re battle them after a time and there are visual clues as to where Micromon are in the grass.
There is an online battle arena and along with capturing all the Micromon and I'd say this is where IAP would get tempting to purchase. Trying to get a group of high DR(Development Rate 1-10, dictates their stat growth rate as they level) Micromon on the rare end of the scale through normal means would require a lot of luck and patience not only getting one to appear but to them capture it since you only get 3 attempts per battle.
Just outside the starting place of the game
There were hints throughout the game that there could be or planned to add more areas in later on to the 4 main areas that are already there.
Overall while short it was a fun, nice looking small Pokèmon like game. Definitely the best one outside of an actual Pokèmon game I've come across and it's helped damped the cravings for an actual Pokèmon game while only spending a fraction the of the price.